With a conventional optical disk apparatus, the reproduction of a signal is performed by irradiating an optical beam having a constant and relatively low light intensity on an optical disk that is an information carrier, and detecting reflected light modulated by an information recording surface of the optical disk as high/low light intensities. In addition, when recording a signal, information is written on a recording material film on the optical disk by modulating an optical beam as high/low light intensities according to the signal to be recorded.
Recording information on an optical disk or reproducing information recorded on an optical disk requires focusing control in which an optical beam is controlled in a normal direction (hereinafter referred to as a “focusing direction”) of a face of an optical disk 1 so that the optical beam is constantly in a predetermined focused state on the recording material film as well as tracking control in which the optical beam is controlled in a radial direction (hereinafter referred to as a “tracking direction”) of the optical disk so that the optical beam is constantly positioned on a predetermined track. Among the aforementioned, tracking control involves detecting high/low levels of reflected light from an information recording surface including tracks of an optical disk as a tracking error signal, and based thereon, moving an objective lens for collecting the optical beam on the optical disk in a tracking direction.
Incidentally, a track of an optical disk is formed in a spiral-shape on an information recording surface. In the case of a read-only disk, a row of pits is formed in a spiral shape. Additionally, in the case in which the optical disk 1 is a recordable or a readable disk, a guiding groove is formed in a spiral shape, and an optically recordable or readable material film is further formed using a method such as deposition on a substrate surface on which irregularities are formed by the guiding groove.
Normally, in the manufacturing of optical disks, a process for fabricating pits or a guiding groove including track information on a reflection surface of light from an optical head 10 by press working or the like is independent from a subsequently-performed process for fabricating a central hole for clamping.
Therefore, the center of the spiral of a track is not completely consistent with the center of the central hole of the optical disk, thereby causing eccentricity during clamping of the optical disk.
Such an eccentricity acts as a primary periodic disturbance component of the optical disk apparatus and significantly reduces following performance with respect to tracks.
Particularly, in recent years, optical disks 1 capable of high density recording in order to record large volumes of information including visual information as well as optical disk apparatuses capable of recording or reproducing information from such optical disks at high speed have come into use. In other words, there is a prominent trend towards higher speed factors. Since the greater the reproduction speed of an optical disk the greater the influence of an eccentricity, such an eccentricity must be effectively compensated in order to accurately follow a track at a high speed factor.
In consideration thereof, control of a movement of an objective lens in a tracking-direction for the purpose of compensating disk eccentricity is being performed independently of normal tracking control (for example, refer to Japanese Patent Laid-Open No. 2005-182968).
FIG. 16(a) is a block configuration diagram of an optical disk apparatus which performs such control.
A configuration and operations of a conventional optical disk apparatus will now be described with reference to FIG. 16(a). As shown in FIG. 16, an optical head 10 is mounted with a semiconductor laser 11, a collecting lens 13, a beam splitter 12, an Fc actuator 14, a Tk actuator 15 and a light detector 16. An optical beam generated from the semiconductor laser 11 passes through the beam splitter 12 and is focused above a disk-shaped optical disk 1 by the collecting lens 13.
The optical beam reflected off the optical disk 1 once again passes through the collecting lens 13 and is reflected off the beam splitter 12 and irradiated to the light detector 16. The collecting lens 13 is supported by an elastic body (not shown) and moves in a focusing direction due to an electromagnetic force when an electric current is passed through the Fc actuator 14. The collecting lens 13 moves in a tracking direction due to an electromagnetic force when an electric current is passed through the Tk actuator 15. The light detector 16 generates a signal according to a detected light intensity.
The light detector 16 sends the signal to an LE signal generator 30. Using the light intensity signal of the light detector 16, the LE signal generator 30 computes a position signal (lens shift error signal: hereinafter referred to as an “LE signal”) with respect to the optical head 10 of the collecting lens 13, and sends the same to an LE signal filter 37 via a subtractor 35.
As an LE signal detection method, an LE signal can be generated by performing a sample/hold or the like on a push-pull tracking error detection signal acquired from a reflected light from the optical disk 1.
In this case, the LE signal generator 30 uses a top hold push-pull method in which are acquired respective pick/hold signals of signals from which push-pull method tracking error signals are obtained by mutual subtraction processing, whereby the pick/hold signals are subtracted from each other. Due to such a configuration, the influence of reflected light from the tracks is eliminated. (For example, refer to Japanese Patent Laid-Open No. H09-274726).
In response to a signal from the LE signal generator 30, the LE signal filter 37 generates a signal for performing lens control and sends the same to a drive circuit 73, whereby the drive circuit 73 drives the Tk actuator 15. A disk motor 20 rotates the optical disk 1, generates a predetermined pulse signal (hereinafter referred to as an “FG signal”) during one rotation and sends the same to a rotational phase detector 21.
The rotational phase detector 21 generates a rotational phase of the optical disk 1 by counting FG signals from the disk motor 20 and sends the same to an LE signal memory regenerator 32 and an LE signal memory recorder 31.
Every time rotational phase information from the rotational phase detector 21 changes during an eccentricity correction recording operation, the LE signal memory recorder 31 records an LE signal of the LE signal generator 30 for each rotational phase on the LE signal memory 33.
The LE signal memory 33 retains an LE signal level for each rotational phase. Every time rotational phase information from the rotational phase detector 21 changes during an eccentricity correction recording operation, the LE signal memory regenerator 32 selects an LE signal level corresponding to the rotational phase from the LE signal memory 33 in synchronization to the change and sends the selected LE signal level to the subtractor 35. The subtractor 35 subtracts the signal from the LE signal memory regenerator 32 from the signal from the LE signal generator 30 and sends the subtracted signal to the LE signal filter 37.
Upon receiving the signal of the subtractor 35, the LE signal filter 37 generates a drive signal whose target value is the stored information of the LE signal memory 33. The drive circuit 73 receives the drive signal and passes a current through the Tk actuator 15, and moves the Tk actuator 15 in a tracking direction. Consequently, the optical beam is adapted to follow the eccentricity of the optical disk 1.
As shown, with the optical disk apparatus shown in FIG. 16(a), since lens control is performed with feedback using a lens shift amount from the LE signal generator 30, eccentricity correction can be performed in a stable manner.
Incidentally, the aforementioned lens control with feedback using a lens shift amount mainly becomes necessary when the optical head 10 performs a long-distance seek that strides a plurality of tracks at one time, and is performed when drive signals inputted to the drive circuit 73 are switched by an operation of a change-over switch 72. Normal tracking control is performed as follows. That is, the light detector 16 detects reflected light from a pit row or the like formed on an information recording surface of the optical disk 1. A TE signal generator 70 generates a tracking error signal (hereinafter “TE signal”) based on the signal detected by the light detector 16. Upon receiving the TE signal, a TE signal filter 71 generates a drive signal based thereon and sends it to the drive circuit 73. The drive circuit 73 passes a current in accordance with the drive signal and moves the Tk actuator 15.
However, the present inventors have discovered the following problems in an optical disk apparatus according to the conventional art described above. With the optical disk apparatus shown in FIG. 16(a), the rotation of the optical disk 1 is a continuous quantity. In contrast, since limits exist on the capacity of the LE signal memory 33 or on the overall processing speed of eccentricity correction, rotational phase information from the rotational phase detector 21 is given as a discrete value sampled according to a count value (sampling frequency) of an FG signal.
A description will now be given with reference to FIGS. 17(a) to (c). It should be noted that, in each drawing, the graph ordinate represents positional changes of the collecting lens 13 while the abscissa represents rotational phases of the disk motor 20.
As shown in FIG. 16(a), while an LE signal representing a positional change of the collecting lens 13 based on an eccentricity of the optical disk 1 assumes a continuous sinusoidal form as shown in FIG. 17(a), a signal from the LE signal memory regenerator 32 assumes a staircase form corresponding to a count value t of an FG signal as shown in FIG. 17(b). At this point, a rising edge portion or a trailing edge portion of steps of the staircase waveform of the signal from the LE signal memory regenerator 32 includes a high-frequency band signal component. Therefore, even when a difference between both signals is obtained by the subtractor 35, the signal waveform of the difference assumes a staircase form.
Meanwhile, as shown in FIG. 16(b), the LE signal filter 37 for performing lens control is configured as a PID filter in which a proportional filter 37a, an integral filter 37b and a differential filter 37c respectively perform a proportional operation, an integral operation and a differential operation in parallel on an input signal.
When a signal from the subtractor 35 which includes a component of a staircase waveform signal is inputted to a PID filter arranged as described above, a drive signal generated after processing also assumes to have a staircase waveform. However, due to the influence of the differential operation performed by the differential filter 37c of the LE signal filter 37, the drive signal is to be outputted to the drive circuit 73 as a signal having a noise-amplified waveform such as that shown in FIG. 17(c). At this point, drive noise appears as a distortion of a rising edge or a trailing edge of steps of the signal waveform.
Particularly, when the gain of the LE signal filter 37 is increased in order to follow an eccentricity of the optical disk 1 with high accuracy, drive noise increases and a risk exists that the threshold set to the drive circuit 73 and which is shown in the drawing is exceeded. When the threshold is exceeded, the drive circuit 73 may become saturated or reactive power may increase. A saturation of the drive circuit 73 causes the waveform of the drive current actually passing through the Tk actuator 15 to differ significantly from the original waveform and prevents stable eccentricity correction operations.
In addition, feedback control is operated using a signal from the LE signal generator 30 in order to perform lens control, but the following problem exists when the LE signal generator 30 uses a top hold push-pull method for detecting a lens shift amount. That is, when an optical beams crosses between tracks of an optical disk 1, and when the crossing speed of the optical beam with respect to tracks of the optical disk 1, i.e., the movement speed in a tracking direction is low, the influence of the reflected light from the tracks is not sufficiently eliminated and an error occurs in the LE signal from the LE signal generator 30. The error disturbs lens control and degrades following accuracy with respect to an eccentricity. A decline in following accuracy impedes stable eccentricity correction operations.
The present invention has been made to solve the problem described above, and an object thereof is to provide an optical disk apparatus and the like which performs stable eccentricity correction operations.